1. Field of the Invention
The present invention relates to a microphone having a MEMS capacitor.
2. Description of the Related Art
In recent years, there have been manufactured small microphones to be mounted in a small electronic instrument such as a mobile phone, an IC reorder, etc. with using the MEMS (micro electro mechanical systems) technique. Japanese Patent Application “Kokai” No. 2007-329560 discloses such a microphone having a capacitor made with the MEMS technique. In this microphone, the MEMS capacitor and a converter circuit comprised of an IC (integrated circuit) are mounted on a common rigid substrate, and over this board, a case having an acoustic hole is placed to complete the microphone. The rigid substrate is formed of a rigid material whose shape does not deform flexibly. The back chamber of the MEMS capacitor is formed within a space created between the MEMS capacitor and the rigid substrate. Further, on the outer face of the rigid substrate (i.e. the opposite face to the face mounting the IC etc.), there is provided a microphone terminal for establishing connection between the electronic instrument or the like and the microphone.
The microphone disclosed in Japanese Patent Application “Kokai” No. 2007-329560 is a top acoustic hole type microphone with the acoustic hole 99 on the opposite side away from the microphone terminal 10t as shown in FIG. 9 accompanying the present detailed disclosure. However, as shown in FIG. 10 accompanying the present detailed disclosure, there exists also a bottom acoustic hole type microphone having the acoustic hole 99 not on the side of the case, but on the side of the rigid substrate 92 (i.e. on the side of the microphone terminal 10t) in order to restrict intrusion of dust or other foreign substance. In this case too, basically, the back chamber of the MEMS capacitor is formed within the space created between this MEMS capacitor and the rigid substrate.
Japanese Patent Application “Kokai” No. 2007-81614 discloses a microphone wherein in a substrate mounting a MEMS capacitor and an IC chip, an acoustic hole is provided immediately below a trench portion of the MEMS capacitor. In this case, conversely from the arrangements shown in FIGS. 9 and 10, the interior of the housing member excluding the trench portion is configured to function as the back chamber of the MEMS capacitor. However, with the top acoustic hole type microphone of this type, it is not possible to provide the microphone terminal on the rigid substrate mounting the MEMS capacitor and the IC chip. Therefore, the entire housing member is comprised of rigid substrates, through which mechanical connection is provided through a conductive member, whereby there is established electrical connection between the rigid substrate mounting the MEMS capacitor and the converter circuit and the rigid substrate mounting the microphone terminal.
With the above microphones disclosed in Japanese Patent Application “Kokai” No. 2007-329560 and Japanese Patent Application “Kokai” No. 2007-81614, both the MEMS capacitor and the converter circuit are mounted on a same rigid substrate. Therefore, there is necessarily imposed restriction on the layout of the MEMS capacitor within the housing member. With the microphone such as the one disclosed in Japanese Patent Application “Kokai” No. 2007-329560 in particular, since the microphone terminal for the electrical connection with an electronic instrument or the like is provided on the outer face of the rigid substrate, the disposing position of the rigid substrate of the microphone is determined, depending on the disposing position of this microphone terminal. And, the disposing position of the substrate imposes restriction on the layout of the MEMS capacitor inside the housing.
With the microphone disclosed in Japanese Patent Application “Kokai” No. 2007-81614, the housing as a whole is comprised of rigid substrates and the connection is made via a conductive member, thus allowing separation between a rigid substrate mounting the capacitor and the converter circuit and a rigid substrate including the microphone terminal. However, with this connection arrangement comprising two rigid substrates electrically connected to each other, as the connection method is mechanical, the electric resistance is high, so that decay of signals, reduction in the noise resistance can be invited.
Further, the acoustic chamber of the MEMS capacitor affects the acoustic characteristics. Hence, if restriction is imposed on the layout of the MEMS capacitor, in the case of the microphone disclosed in Japanese Patent Application “Kokai” No. 2007-329560 in particular, the size of the back chamber too is restricted. That is, the size of the back chamber depends on the size of the trench formed in the MEMS capacitor, so that the acoustic characteristics of the microphone will largely depend upon the performance of the MEMS capacitor. As a result, it becomes difficult to employ a common MEMS capacitor in various microphones having various housings. As a result, it will become difficult to achieve mass production cost reduction advantage. FIG. 10 in Japanese Patent Application “Kokai” No. 2007-329560 shows an example wherein the back chamber is enlarged by cutting away the substrate (rigid substrate). This arrangement, however, has may disadvantages in the respect of production cost.
Therefore, there is a need for a technique that allows layout of a MEMS capacitor with large degree of freedom without inviting any deterioration in the electrical performance such as signal decay, noise resistance reduction and that allows using of a same MEMS capacitor in various microphones having a variety of housings.